A spark plug is conventionally known, in which a tip is welded to a ground electrode for improvement of spark wear resistance. Japanese Laid-Open Patent Publication No. 2004-134136 discloses a method for manufacturing such a type of spark plug, including the step of detecting a position or displacement amount of the ground electrode that can affect the ignition performance of the spark plug. In this method, the position or displacement amount of the ground electrode is detected by taking an image of an end face of the ground electrode with the use of reflected light from the spark plug (work) and performing image processing on the taken image.
The above-disclosed method has the problem that, when the color and gloss of the end face of the ground electrode are varied under the influence of heat during welding, there occurs variations in color and gloss between plug works so that the accuracy of detection of the ground electrode by image processing becomes deteriorated.